It is effective to rise a combustion temperature for increasing an efficiency of a gas turbine. Therefore, a Ni-based heat resistant alloy having high-temperature strength is employed for various parts in components for the gas turbine. The Ni-based alloy contains large amounts of solid-solution strengthening elements such as tungsten, molybdenum or cobalt and precipitation strengthening elements such as aluminum, titanium, niobium or tantalum, and therefore has excellent high-temperature strength. A γ′ phase (Ni3Al) is a main precipitation strengthening phase, and the strength thereof increases depending on an increase of its temperature. Thus, the γ′ phase is very effective in improving the strength at a high temperatures. The γ′ phase is stabilized by adding elements of titanium, niobium, tantalum or the like, and thus is capable of existing at a higher temperature. Therefore, development has been carried out by placing a focus on how the γ′ phase is stabilized in order to improve a performance of the Ni-based alloy. The added elements such as titanium, niobium, tantalum or the like are liable to produce segregation during solidification, and thus make it difficult to manufacture a large-sized component. Thereupon, the high-strength Ni-based alloy is restricted to a relatively small-sized component such as a rotor/stator blade.
A turbine disc is raised as a typical large-sized component of the gas turbine. A high-chromium ferritic heat resistant steel has been conventionally used as the turbine disc material. Recently, however, the Ni-based alloy has come to be employed due to a demand for use at higher temperatures. A large size ingot exceeding 10 tons has been produced for a heat resistant steel. As for the Ni-based alloy, Alloy 706 having relatively high productivity has been used for the turbine disc. However, a weight thereof is limited to 2 to 3 tons. If the product weight exceeds the value, segregation occurs at the time of production, and cracks are undesirably generated at the time of solidification or at the time of forging, so that it is difficult to produce a larger product.
Alloy 718 is a Ni-based alloy having a strength higher than that of Alloy 706, (see JP-A-2000-192208 and JP-A-2000-192179). However, Alloy 718 contains large amounts of the above-described precipitation strengthening elements to achieve the higher strength, so that segregation is liable to occur as compared with Alloy 706, and it is difficult to produce a large-sized steel ingot. Therefore, it is difficult at present to use Alloy 718 for a large-sized gas turbine.